Acoustic device



F. F. ROMANOW ACOUSTIC DEVICE Feb. 1, 1938.

Filed June 9, 1934 5 Sheets-Sheet 1 FIG. 4

' IN VE/V TOR E /-T ROM/WOW 0mm 6. 7M

A TTORNEY RESPONSE IN DB Feb. 1, 1938. F. F. ROMANOW ACOUSTIC DEVICEFiled June 9, 1934 3 Sheets-Sheet 2 IOOOO .5600

FREQUENCY IN CYCLES PER SECOND lNl/EN TOR F. /-T ROMANOW ATTORNEY Feb.1, 1938.

RESPONSE IN DB RESPONSE IN DB F. F. ROMANOW 2,106,813

ACOUSTIC DEVI CE Filed June 9, 1934 I5 Sheets-Sheet 3 ATTORNEY PatentedFeb. 1, 1938 UNITED STATES PATENT orrics ACOUSTIC DEVICE YorkApplication June 9, 1934, Serial No. 729,759

Claims.

This invention relates to acoustic devices, and,

more particularly, to microphones or trans mitters employed in opensound fields.

The open field characteristic of most pres- 5 sure operated microphonesshows a considerable increase at the high frequencies. This is caused byseveral factors which includes the diffraction around the microphonecasing or housing, resonance in any cavity in front of the diaphragm ofthe microphone, and resonance in the holes of the diaphragm-protectingscreen or guard, if one is used. The cavity in front of the diaphragm isformed by parts of the microphone necessary to hold the diaphragm inposition, although effort is made to keep this cavity as shallow aspossible. The resonant effects are independent of the angle of soundincidence, while diffraction decreases considerably as the microphone isrotated around an axis lying in the face of the instrument. For anyposition other than normal sound incidence, the pressure actuating themicrophone is out of phase for different points of the diaphragm, thuscausing an additional loss with the result, therefore, as the microphoneis 5 rotated, a rapid decrease in response. It would be of greatadvantage to have a microphone or transmitter which would be relativelyfree of these angle effects and show a uniform response regardless ofthe angle at which it is placed in 30 the sound field with reference tothe source of sound waves.

An object of this invention is to improve the open field responsecharacteristic of a microphone.

5 A feature of this invention comprises a microphone that may be placedat any angle with reference to a sound source and have a substantiallyuniform response.

A further feature of this invention comprises 40 a microphone includingmeans in front of and spaced from the diaphragm thereof for impedingsound waves incident normally, but allowing free access of sound wavesincident 90 to 180 removed from those incident normally.

45 Another feature comprises means for controlling the extent to whichthe high frequencies in the speech and music range have access to thediaphragm of a microphone.

Still another feature comprises means'adjust- 50 ably positioned infront of the diaphragmforcontrolling the high frequency response of a.microphone.

Other and additional features will be evident from the general anddetailed description which follows hereinafter.

This invention may be embodied in an acoustic device comprising a casingsupporting a diaphragm and housing electrical means associated with thediaphragm for the translation of sound waves into electrical signals.Supported by, and 5 adjustable to and from the casing, is a screenmember, preferably of greater area than the diaphragm, and having itsmarginal edge outside of that of the diaphragm. The screen comprises,preferably, one or more thicknesses of silk cloth 10 of predeterminedacoustic resistance, preferably such as to impede the passage to thediaphragm of sound waves incident substantially normally. The screen issufliciently spaced from the diaphragm to provide a substantiallyannulariateral 15 passage permitting sound waves free accesstherethrough to the diaphragm.

A more complete understanding of this invention will be obtained fromthe detailed description which follows, read with reference to the 20appended drawings, wherein:

Fig. 1 is a side elevational view of an acoustic device embodying thisinvention;

Fig. 2 is a front elevational view of the device of Fig. 1, partiallybroken away;

Fig. 3 is an exploded perspective view of the diffraction-effectcounteracting means embodied in the device of Fig. 1 in accordance withthis invention; I

Fig. 4 is a side elevational view of the microphone embodied in thedevice of Fig. 1, partly broken away and partly in section;

Fig. 5 is a side elevational view of another acoustic device embodyingthis invention, partly broken away;

Fig. 6 is a front elevational view of the device of Fig. 5, partlybroken away;

Fig. 7 shows a series of frequency response curves for the microphone ofthe acoustic device of Fig. 1 for different angles of sound incidence;

Fig. 8 shows a series of frequency response curves for the acousticdevice of Fig. 1 for the same angles of sound incidence; and

Fig. 9 shows a series of frequency response curves for a modification ofthe device of Fig. 1.

The acoustic device of Figs. 1-4 comprises a microphone or transmitterID, of the pressure operated type, specifically of the electrostatictype. The microphone comprises a casing H having a cylindrical frontportion l2 having an open end is and a frusto-conical portion H. Adiaphragm 15, for instance, of a light weight, high strength material,such as aluminum or an aluminum alloy, such as duralumin, and preferablystretched, is mounted in the open end of the case, being secured theretoat its periphery by any suitable means, such as the clamping ring l6,and has one or its outer surface exposed to sound waves. A stationaryelectrode I1 is supported within the case and closely spaced to theinner or rear and enclosed surface of the diaphragm.

The open field response of such an acoustic device provided with acylindrical extension l8 for housing an amplifier for different anglesof sound incidence, is shown by Fig. 7. It will be noted particularlythat as the angle of incidence changes from to 90, the frequencyresponse at the high audio frequencies, particularly between 2000 and15,000 cycles per second, varies considerably and is greatly reducedfrom that of normal incidence. Between 90 and 180, the variation issmall and the reduction is only slightly greater than that for 90incidence. These variations in response are ascribable to variousfactors. The cavity IS in front of the diaphragm, although kept asshallow as possible, provides'a resonance that tends to increase theresponse in the upper frequency range. The greater portion, however, ofthe increase in response at the high frequencies results fromdiffraction around the microphone casing. While the cavity resonance isindependent of the angle of sound incidence, the diffraction effectsdecrease considerably as the microphone is rotated around an axis lyingin the face of the instrument, assuming a substantially planar face. Forany position other than that for normal sound incidence, however, thesound wave pressure is out of phase for different points on thediaphragm, causing an additional and rapid decrease in the highfrequency response.

A microphone having a substantially uniform response regardless of theangle at which it is positioned in the sound field with reference to thesound source, may be attained in accordance with this invention byembodying in the microphone |0, a sound wave impeding or sound wavefrequency discriminating member or screen 20 spaced from and in front ofthe diaphragm l and of such size that its periphery or marginal portionis outside of that of the diaphragm and the open end of the casing. Thescreen is preferably of one or more thickness of silk cloth. It ismounted at its periphery between a pair of rings 2|, the apertures inwhich contain metallic gauze screens 22, providing a mechanicalprotection for the silk cloth, the rings 2| and the screen 20 beingsecured together by any suitable means, such as the screws 23 and nuts24. A sleeve or ring member 25, encircles the casing portion i2, and isslidable thereon. A plurality of spacers comprising thin, narrow, rigid,me-v tallic strips 21 are fastened at one end 28 to the sleeve 23 and attheir other ends 29 to one of the rings 2|, and are separated at regularintervals. By this arrangement, the screen 20 may be adjusted to andfrom the microphone until the position for optimum results is obtained.

' Of course, this optimum spacing could be predetermined and the screenand its support secured rigidly and immovably to the microphone, thesupport being constructed as a part of the microphone casing. In orderto attain the optimum results. that is, .a substantially uniformresponse regardless of the angle of sound incidence, the screen. shouldbe atleast of such dimensions that if ,the front end of the microphonecasing were'placedagainst the screen, its peripheral or marginal portionwould be within,

that of the screen, i. e., the screen should be at least as, butpreferably more extensive in all directions laterally than the diaphragmor the front end of the microphone casing; its acoustic resistanceshould be such that the phase change in the sound waves passing throughthe screen to the microphones diaphragm, results in a diminution in thepressure on the diaphragm to a value below that which would exist if thescreen were not there; and its distance from the front end of the casingand, therefore, from the diaphragm, should be such that the equalizingof the response takes place particularly in the frequency region inwhich the greatest variations in response would take place withvariation in angle of sound incidence without the use of the screen.Placing the screen nearer to the diaphragm than for optimum results,causes a greater decrease in the effect of diffraction at the upper endof the high frequency region of the audio frequency range, while placingit farther away causes a greater decrease in the effect of diffractionand angle effects at the lower end of the high frequency region of theaudio frequency range.

Fig. 8 shows a series of frequency response curves for different anglesof incidence for the device of Figs. 1 to 4. The portion l2 of themicrophone casing was approximately one inch in diameter, and a singlelayer of silk cloth two and one-half inches in diameter and having anacoustic resistance of H mechanical ohms per square inch constituted thescreen 20, which was positioned at a distance of approximatelythreeeighths of an inch from the front end of the casing. It is to benoted that at least up to 10,000 cycles per second, the response forangles of incidence between 0 and 180 varies by not more than 5decibels. An even more uniform response than that evidenced by Fig. 8,was obtained when the single layer of silk was replaced by two layers ofsilk. This is shown by Fig. 9. It is clear that sound waves incidentnormally, and at least a portion of the sound waves incident at an angleup to 90, reach the diaphragm after passing through the screen 20, whilesound waves l incident between at least 90 and 180 reach the diaphragmof the microphone either directly through the substantially annularpassage 30 defined by the microphone casing and the screen 20 or byreflection from the screen 20.

Figs. 5 and 6 show another acoustic device embodying this invention. Itis similar in most respects to the device of Figs. 1 to 4, like partsbeing indicated by like reference characters. Instead of the screen 20,a screen 3|, performing the same function, may be employed. This screen3| comprises, preferably, a disc or plate of insulating material such asglass, hard rubber, or phenol condensation product, or corrosion re-'sistant metal, such as aluminum or duralumin,

containing a multiplicity of passages 32, each a reference to variousspecific embodiments thereof,

it is to be understood that modifications may be made therein withoutdeparting from the inven tion, which is to be considered as limited,therefore, by the appended claims, only.

Whatisclaimedis: 7

1. A- transmitter comprising a casing having an opening therein, adiaphragm mounted in said casing and juxtaposed to said opening, anacoustic screen of uniform fabric in fixed axial alignment with and infront of said diaphragm and spaced from the wall of said casing todefine an open lateral space "between said screen and said casing, andmeans including a plurality of spacer members for supportingsaid screen.

2. A non-directional transmitter comprising a casing having an openingtherein, a diaphragm mounted in'said casing adjacent said opening,- afabric screen of uniiorm acoustic impedance characteristic over itsentire area, mounted infront of said diaphragm and opening and in fixedaxial relation thereto by a pair of clamping members between which theperipheral portion of said screen is secured, and a plurality of spacermembers extending between the clamping members and the casing wherebythe transmitter responds uniformly to sound waves'from all directions.

3. A transmitter, comprising a casing having an opening therein, adiaphragm mounted in thecasing adjacent said opening, a silk fabricscreen of uniform acoustic impedance characteristic over its whole area,which is greater than that of the diaphragm, mounted in front of saiddiaphragm and opening in spaced relation thereto and in .fixed axialalignment therewith by a pair of clamping members between which theperipheral portion of said screen is secured, and a plu-,

rality oi spacer members extending between said clamping members and thecasing, thereby defining a fixed, uniform, substantially annular openingbetween the screen and casing, whereby the transmitter respondssubstantially uniformly to sound waves arriving from all directions.

4. A transmittingdevice comprising a casing having an opening, adiaphragm mounted in the casing adjacent the opening, an acousticimpedance means and supports attached to said casing for mounting saidmeans in spaced relahaving a diaphragm and an acoustic impedance screenof fabric, having uniformly spaced weft and warp threads, mounted infront of said 'diaphragm in spaced relation thereto and in fixed axialalignment therewith, whereby the acoustic impedance losses of soundwaves from the front are substantially equivalent to the diiiraction.

losses 01' sound waves from the sides and back oi the device. 1 o

